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Wednesday, February 20, 2013

UseDNS no in sshd_config

"It turns out there is a setting in OpenSSH that controls whether SSHd should not only resolve remote host names but also check whether the resolved host names map back to remote IPs. Apparently, that setting is enabled by default in OpenSSH. The directive UseDNS controls this particular behaviour of OpenSSH, and while it is commented in sshd_config (which is the default configuration file for the OpenSSH daemon in most enviornments), as per the man page for sshd_config, the default for UseDNS is set to enabled. Uncommenting the line carrying the UseDNS directive and setting it to “no” disables the feature."

So to speed up connection speed of OpenSSH, in /etc/ssh/sshd_config file, put

UseDNS no

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